...do you have the repair guides? , it says the reset cap may be at fault C1 (47uf 16 volt) or the reset transistors .
It mentions the test button  on S/B sometimes causing the board to lock up to , if you have problems with the sound stuttering. turn game off then on to reboot...

Thanks, yes, I have the repair guides, I just read the section you are referring to. I may try replacing C1. Do you know which are the reset transistors? Looking at the schematic, it looks like they would be Q3, Q4 and Q5

I don't think it is the test button, I haven't had stuttering and have rebooted the game many times.

The Bally repair guide also mentions the following possible causes for a locked on LED:

Faulty U5, Faulty U15, leaky C1, open R1, leaky CR1 or faulty U17

So I'll probably go through those as well after I have made sure the problem is on the board.